feat: replace MeloTTS with Coqui XTTS-v2 natural Korean voice

MeloTTS's single Korean speaker sounded non-native ("foreign accent"). Swap it
for Coqui XTTS-v2 with the built-in female studio speaker "Ana Florence"
(language ko), the natural voice used in earlier local runs.

- bridge/xtts_worker.py: new warm HTTP worker (own /opt/xtts venv), same
  /synth + /health contract and PCM16 output as the old melo worker
- docker/setup-xtts.sh: builds the venv with cu128 torch (Blackwell) + Coqui
  TTS and bakes the XTTS-v2 model offline. Pins transformers>=4.57,<5 (5.x
  removed isin_mps_friendly, breaking XTTS) and installs the [codec] extra
  (torch>=2.9 needs torchcodec) — both verified by a real host synth
- Dockerfile: replace the melo build layer with the xtts layer
- supervisord.conf: melo-worker -> xtts-worker, env passthrough for
  XTTS_DEVICE/SPEAKER/LANGUAGE (always set via compose defaults)
- bridge/server.py: default TTS_ENGINE=xtts, route to the xtts worker, generic
  worker-synth helper, neural-only fallback flag (XTTS_FALLBACK_PIPER)
- settings UI: engine dropdown xtts/piper, drop the dead melo_speed field, fix
  the supervisorctl restart target to xtts-worker
- compose/.env.example/README: XTTS_* vars, speaker/language knobs, remove melo
- remove bridge/melo_worker.py and docker/setup-melo.sh
- tests: xtts treated as multilingual (not English-only)

Verified on host: coqui-tts loads XTTS-v2 and synthesises Korean as
"Ana Florence" to a 16-bit mono 24kHz WAV.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -65,18 +65,19 @@ RUN ls -d /opt/venv/lib/python*/site-packages/nvidia/cublas/lib \
         > /etc/ld.so.conf.d/nvidia-cu12.conf 2>/dev/null \
     && /sbin/ldconfig || true
 
-# --- MeloTTS Korean voice (separate /opt/melo py3.11 venv; see setup-melo.sh).
-#     Heavy layer (torch CPU + transformers + MeCab); placed before the app
-#     COPY so it stays cached across source-only changes. ---
-COPY docker/setup-melo.sh /app/docker/setup-melo.sh
+# --- Korean voice: Coqui XTTS-v2 (separate /opt/xtts py3.11 venv; see
+#     setup-xtts.sh). Natural female Korean ("Ana Florence"); replaces MeloTTS.
+#     Heavy layer (torch cu128 + Coqui TTS + the baked XTTS-v2 model); placed
+#     before the app COPY so it stays cached across source-only changes. ---
+COPY docker/setup-xtts.sh /app/docker/setup-xtts.sh
 # Strip CR before running: a Windows checkout (autocrlf) yields CRLF, which makes
-# bash read line 18 as `set -euxo pipefail\r` and abort with
-# "set: pipefail: invalid option name". .gitattributes pins *.sh to LF, but this
-# keeps the build working even on a not-yet-renormalised working tree.
-RUN sed -i 's/\r$//' /app/docker/setup-melo.sh && bash /app/docker/setup-melo.sh
+# bash read `set -euxo pipefail\r` and abort with "set: pipefail: invalid option
+# name". .gitattributes pins *.sh to LF, but this keeps the build working even on
+# a not-yet-renormalised working tree.
+RUN sed -i 's/\r$//' /app/docker/setup-xtts.sh && bash /app/docker/setup-xtts.sh
 
 # --- Human input + window management for the on-screen Chrome control tool.
-#     Placed AFTER the heavy melo layer so it doesn't bust that cache. xdotool
+#     Placed AFTER the heavy TTS layer so it doesn't bust that cache. xdotool
 #     injects real X pointer/keyboard events (visible cursor, char-by-char
 #     typing) into the broadcast; wmctrl lists/moves windows. ---
 RUN apt-get update && apt-get install -y --no-install-recommends \